Acid lateritic soil and groundnut on a Kandhamal upland
Susmita Pradhan has 1.0 ha of red lateritic upland in Kandhamal, Odisha. Soil test: pH 4.9, organic carbon 0.42%, available N 180 kg/ha, P2O5 9 kg/ha, K2O 135 kg/ha, exchangeable calcium low, and the laboratory flags high exchangeable aluminium. She plans to grow bunch groundnut (cv. Devi, 105 days) in the coming kharif and has arranged 100 kg DAP, 40 kg MOP, and 2 t agricultural lime. To save labour, she intends to broadcast the lime and DAP together just before sowing and incorporate both in one pass. She has no gypsum and asks whether it is necessary. Rainfall is about 1,200 mm, and the field slopes gently, so runoff is possible. Last year's finger millet yielded poorly, and she blames the soil. She wants a single recommendation that will fix the soil in one season.
1. Interpret the soil test for groundnut. Which two constraints should be addressed first, and why?
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The soil test shows three interacting problems: strong acidity (pH 4.9), high exchangeable aluminium, and low available phosphorus (9 kg P2O5/ha) with low calcium. For groundnut, aluminium toxicity damages root tips and restricts nodulation and calcium uptake, while low P limits pod development. The two constraints to address first are acidity/aluminium toxicity and calcium supply; phosphorus is third because P is also fixed by aluminium and iron at this pH. Potassium is adequate. Organic carbon is low, so biological activity and nutrient cycling are poor. Do not try to fix everything in one season: raising pH from 4.9 to about 6.0 is a multi-season process. The first intervention is lime incorporated into the plough layer, followed by banded P at sowing; gypsum is a later, targeted calcium and sulphur input, not a substitute for lime.
2. Evaluate her plan to mix lime and DAP. Give a corrected lime and fertilizer schedule with rates, timing, and placement.
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Her plan to mix lime and DAP is agronomically wrong. Lime raises pH sharply in the immediate zone; when mixed with DAP, it converts ammonium to ammonia (N loss) and precipitates calcium phosphates, reducing P availability. Lime should be broadcast and incorporated 15–20 cm deep at least 3–4 weeks before sowing, not in the same pass as fertilizer. With 2 t/ha agricultural lime and a pH target near 6.0, apply the full 2 t/ha now and re-test after harvest; if the laboratory's lime requirement is higher, split it over two seasons. At sowing, place DAP in furrows 5–7 cm below and slightly to the side of seed, at 100 kg/ha (about 18 kg N and 46 kg P2O5/ha), and apply 40 kg MOP/ha basal. Do not broadcast DAP over the lime. Add well-decomposed FYM to improve structure and moisture retention.
3. Advise on gypsum: is it needed, when, at what rate, and how would you justify the cost to her?
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Gypsum is useful but not a pH corrective. Groundnut has a high calcium demand at pegging and pod filling; on this calcium-poor, aluminium-affected soil, 250–400 kg gypsum/ha applied at flowering over the pegging zone can improve pod filling and kernel quality and supplies sulphur. Apply it after the crop has pegged, broadcast over the row, and water it in if possible. Justify the cost by the expected reduction in empty pods and improved kernel grade; if the budget is tight, prioritize the lime, because without lime the gypsum response will be limited. Do not apply gypsum at sowing in place of lime.